1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device having a hetero interface.
2. Description of the Related Art
A Group-II-VI compound semiconductor such as ZnSe and a nitride Group-III-V compound semiconductor such as GaN have a wide bandgap (wide gap) equivalent to or larger than an amount of energy corresponding to the wavelengths of light in the visible wavelength range and have received much attention as a visible light emitting material. Particularly, the operating wavelength range of a semiconductor laser or light emitting diode formed of a Group-III-V compound semiconductor material such as GaAlAs, InGaAlP lies in a wavelength range longer than that of green, but the semiconductor laser or light emitting diode formed by use of the wide gap Group-II-VI compound semiconductor and nitride Group-III-V compound semiconductor such as GaN may be operated in an operating wavelength range of blue or ultraviolet rays having shorter wavelengths. This makes it possible to apply the advantages of the conventional semiconductor light emitting device such as the small size, light in weight, low operating voltage, high reliability in the short wavelength region so that the density of the optical disk can be increased. Further, outdoor message boards can be displayed in full color.
In recent years, continuous oscillation of the current injection type semiconductor laser at room temperatures and high-luminance operation of the light emitting diode are reported in relation to blue-green semiconductor light emitting devices using wide gap Group-II-VI compound semiconductor such as ZnSe. For example, a current injection type blue-green semiconductor laser device using CdZnMgSeS is constructed as follows. That is, an n-type ZnMgSeS clad layer, n-type ZnSe light guide layer, CdZnSe quantum-well layer, p-type ZnSe light guide layer, p-type ZnMgSeS clad layer, p-type ZnSeS, and p-type ZnSe contact layer are sequentially stacked on one main surface of an n-type GaAs substrate. The p-type ZnSe contact layer is processed into a mesa stripe form with a width of 5 .mu.m by chemical etching, an insulating layer is formed on the p-type ZnSeS layer outside the mesa portion and the surface thereof is made flat. A p-side electrode is formed on the flat surface. The p-side electrode is a laminated electrode of Pd/Pt/Au laminated in this order from the bottom. An n-side electrode formed of In is disposed on the other main surface of the n-type GaAs substrate.
It is reported that the blue-green semiconductor laser device with the above structure can effect the continuous oscillation at room temperatures (Room Temperature continuous operation of Blue-Green Laser Diodes, N. Nakayama et. al, Electronics Letters, Vol. 29, No. 16, pp. 1488-1489 (1993)).
In the above conventional cases, blue-green light is obtained by growing Group-II-VI compound semiconductor such as ZnSe, ZnMgSSe on the GaAs substrate by the molecular beam epitaxy (MBE).
In the nitride-series semiconductor laser containing GaN, short wavelengths shorter than 350 nm can be obtained and the oscillating operation at 400 nm is reported. The reliability that the light emitting life is longer than 10,000 hours is confirmed. Thus, nitride semiconductor is an excellent material which satisfies the necessary conditions as an optical disk recording light source for the next generation. However, even with this material, the subject of a lowering in the operating voltage remains.
A semiconductor device for effecting the low-voltage operation as a practical semiconductor laser or light emitting diode (LED) has not been realized.
In the conventional short-wavelength semiconductor light emitting device using ZnSe, it has been considered that the operating voltage is raised by the hetero barrier caused in the hetero interface of the p-type layer or the Schottky contact between the p-side electrode and the p-type semiconductor. However, it is proved as the result of study by the inventors of this application that a voltage drop in the junction between a layer having a large bandgap (for example, n-ZnSe) and a layer having a small bandgap (for example, n-GaAs) in the hetero interface of the n-type layer prevents a lowering in the operating voltage. This is considered to be because the band discontinuity in the conduction band becomes larger depending on the condition of the crystal growth and injection of electrons is prevented. It is desired to reduce the voltage drop in this portion and realize the low-voltage operation of the device by improving the obstruction to injection of electrons by the band discontinuity.
Further, a hetero barrier is present between the substrate and the GaN layer of a laser using GaN. The magnitude of the hetero barrier varies according to the condition of crystal growth, and it is proved by the study by the inventors that the hetero barrier on the conduction band side which has been considered to be low is larger than an expected value. It is necessary to reduce the voltage drop in this portion and realize the low-voltage operation of the device by improving the obstruction to injection of carriers by the band discontinuity.